it doesn't have an impact with online lag... but it limits the filesize of the developers patches... meaning you may only be able to have 1 or 2 patches for games that have updates released on your wii at one time...
The flash card expasions (SD cards) would enable extra space and backups for this, however it may interfer with load times and whether the community would spill out cash for the larger cards. (i bought a 1 gig one for mine at $5 on sale) But if many games start releasing patches we may need up to 10 gigs or more depending on your game collection and the filesizes of the patches addressed by the games' developers. meaning you'd have to get a larger one and manually transpher the patches or redownload them, or have multiple sd cards and have you make sure you have a certain sd card in the game when you played it.
A hard disk expansion would be very possible with the wii because of its usb support... however IDK if the big N would be willing to allow 3rd party external drives to run on their machines. Time will tell, problems and solutions arrive with every new turn and the wii is still an incredible piece of hardware even with all these limitations.
Target Terror + Medal of Honor 2: Heroes + Super Mario Galaxy ~
Resident Evil 4: Wii Edition + Mario Kart Wii + Red Steel + Counter Force ~
Monster 4x4 World Circuit + Battle Rage + Obscure The Aftermath + Manhunt 2 ~
( PM to add me on any Wi-Fi games )